Mechanism of Burning of Fully-Developed Compartment Fires Harmathy, T

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Mechanism of burning of fully-developed compartment fires Harmathy, T. Z.

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Publisher’s version / Version de l'éditeur: https://doi.org/10.4224/40001768 Paper (National Research Council of Canada. Division of Building Research); no. DBR-P-777, 1978

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MECHANISM OF BURNING OF FULLY-DEVELOPED COMPARTMENT FIRES s

by T.Z. Harmathy

Reprinted, from Combustion and Flame Vol. 31, No. 1, 1978 p. 265 273

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I Division of Building Research

Price 25 cents OTTAWA NRCC 16722 SOMMAIRE

La croyance selon laquelle un manque d'air limite la vitesse de d6veloppement d'un incendie 2 ventilation contr816e est trPs rhpandue. Apres examen critique, ce principe ne tient pas. D'autres m6canismes qui pourraient expliquer la faqon dont le dhbit de l'air dans un compartiment contrsle la vitesse de combustion (pyrolyse) d'un mathriau combustible cellulosique sont examinhs et un modsle plausible est prGsent6. L'hypothsse sugg6r6e est que l'oxydation de la surface carbonis6e est le lien qui explique le rapport observg entre le taux de pyrolyse et le taux d1entr6e d'air. Le modsle proposg permet de tirer des conclusions compatibles avec les connaissances actuelles sur les incendies I ventilation contr816e et ceux 2 surface de combustible contrCl6e. Une tentative d'explication du m6canisme de combustion I 11int6rieur d'une enceinte de polym6res synthstiques non carbonisables est 6galement donn6e. Mechanism of Burning of Fully-Developed Compartment Fires

T. Z. HARMATHY Fire Research Section, Division of Building Research, National Research Council of Canada, Ottawa, KIA OR6

It has been widely believed that in ventilation-controlled building fires a shortage of air limits the rate of burning. This concept is critically examined and proved untenable. Other possible mechanisms by which the flow rate of air into a compartment may control the rate of burning (pyrolysis) of cellulosic fuel are examined and a plausible model presented. It is suggested that oxidation of surface char is the link by which the rate of pyrolysis is related to the rate of entry of air. The proposed model provides conclusions compatible with present knowledge of both ventilation-controlled and fuel-surface-controlled fires. It is emphasized, in conclusion, that the model is probably not applicable to compartments in which the fuel consists pre- dominantly of non-charring plastics.

INTRODUCTION combustion of volatiles (a series of gas-phase reactions), and (iii) the oxidation of the solid That the rate of burning of cellulosic fuel in fully- decomposition product, char (a heterogeneous developed compartment fires is, under a wide solid-gas reaction). On the other hand, what fire range of conditions, approximately proportional scientists refer to as rate of burning is, in fact, to compartment ventilation [I, 2, 3, 4, 5, 61 the rate of loss of mass of the fuel associated was probably the first important discovery of fire primarily with the departure of the volatile decom- science. The explanation seemed quite straight- position products and, to a lesser degree, with the forward. The rate of entry of air, it was believed, oxidation of char. This has nothing to do with limits the extent to which oxidation reactions can the rate of the dominant combustion process, approach the stoichiometric relations and, in the combustion of the volatiles, although this turn, limits the rate of heat evolution in a com- process is the chief source of heat evolution in partment. This view, apparently, still persists. the compartment. Clearly, it would be justified Several years ago the author [7] pointed out to replace the expression rate of burning by the that referring to poorly vented fires as "ventila- more appropriate (but still not fully accurate) tion-controlled" is indeed very apt, since the rate term rate of pyrolysis or rate of volatile produc- of entry of air literally controls, not limits, the tion. rate of burning; more exactly, it controls the rate- Although for some fuels (among them all determining link in a complex process hidden cellulosic materials) the nature of the decomposi- behind the simple word burning. tion reactions depends on the presence or absence What, in everyday language, is referred to as of oxygen in the atmosphere, the rate of pyrolysis ~urningis, with cellulosic fuels, a process consisting is controlled largely by the rate of heat supply 3f three entirely different types of reaction: to the decomposing fuel. Consequently, the rate 3) the pyrolysis of fuel into (roughly) 85% volatile of entry of air into the compartment can control md 15% solid decomposition products, (ii) the the rate of pyrolysis (the so-called rate of burning)

Eopyright 01978 by The Combustion Institute published by Elsevier North-Holland, Inc. 0010-2180/78/0031-0265 $1.25 T. Z. HARMATHY only in an indirect way: by being instrumental entry of air and drives the pyrolysis reactions2 I in regulating the rate of heat supply to the fuel. by thermal feedback. Yet this suggestion will not Various mechanisms by which ventilation may stand scrutiny either. One must realize that the control the rate of pyrolysis of cellulosic materials gaseous phase in a burning compartment does not will be critically surveyed and a plausible model consist of a well-stirred mixture of volatiles, air, suggested. The mechanism of pyrolysis (burning) and combustion products; in other words, that of non-charring plastics will also be hypothesized. the conditions in it are far from ideal for gas- phase reactions. In fact, as dlbe discussed later, the flame envelope fairly well divides the gas- CRITICISM OF SOME SIMPLE CONCEPTS filled space into two zones, one consisting mainly Traditionally, the dependence of rate of burning of air at virtually undepleted oxygen content, on compartment ventilation has been explained the other mainly of volatile decomposition by claiming that the rate of entry of air sets an products and combustion gases. Consequently, upper limit to which oxidation reactions can it is as a rule the rate of entrainment of air into the approach the stoichiometric relations. That this flame envelope, not the rate of entry of air into view is untenable can be proved by analysing a the compartment, that controls the rate of com- multitude of compartment burn-out experiments. bustion of the volatiles and thereby possibly An equation developed by Thomas et al. (See Eq. affects the decomposition of the fuel. (2i) in Ref. [5]) reveals that in fully developed, Admittedly, increased ventilation does create ventilation-controlled fires the ratio of the rate improved turbulence along the flame boundaries, of entry of air to the rate of volatile production enhancing the rate of air entrainment into the is approximately 5.41. The author [7] found an flames and, in turn, the rate of combustion of the even higher ratio of rate of air flow to rate of volatiles. Yet any suggestion that the rate of volatile production, 6.13. In comparison, the combustion of the volatile decomposition products stoichiometric air requirement for the volatile might have more than a marginal effect on the decomposition products of a "typical" wood rate of pyrolysis of the fuel can be dismissed, for is only about 4.19 kg air per kg volatilesl. Con- the following reasons. sidering that under fuel-surface-controlled condi- The rate of combustion of the volatile decom tions (to be discussed later) the ratio of air flow position products has a decisive influence on t rate to rate of volatile production is necessarily temperature of the gases and compartment boun higher than 5.41, one can state with fair certainty aries during fire. As heat supply to the decom that the rate of entry of air is sufficient for the posing fuel is expected to take place by hig complete combustion of cellulosic materials within temperature-dependent transfer processes, one the boundaries of the compartment. In spite of this, the appearance of large flames outside the There are contradictory views on whether the pyrol windows of burning compartments is common. of wood is primarily an endothermic or an exothe One may now propose that it is probably the process. According to Roberts [8, 91, und rate of combustion of the volatile decomposition atmospheric conditions the over-all heat of d products, not the degree of completeness of their tion is slightly on the exothermic side. At 1 peratures, however, up to about 320°C, combustion, that is controlled by the rate of fuel "suitable" for the exothermic reactions, heat mus be supplied to its virgin core. In addition to that re The actual air requirement for a burning wood is some- quired to drive the low-temperature endothermi what larger than the given value because a lesser amount reactions, this heat also includes the heat of vapo of char also oxidized simultaneously with the volatile tion of moisture, and the sensible heat required decomposition products. But even if it is assumed that raising the temperature of the core through the range the char is consumed by oxidation as soon as it forms, endothermic reactions. It is not contradictory, the the stoichiometric air requirement is still only 5.11 kg fore, to say that the pyrolysis of wood, at least up to air/kg fuel [7], below the level provided by ventilation. certain temperature, requires a steady supply of heat. led to conclude that the rate of pyrolysis is very of the surface of the decomposing fuel is covered sensitive to the prevailing temperatures and, in with oxidizing char (shown in heavier lines). turn, to the rate of gas-phase reactions. This con- Finally, zone 3(a) contains the part of the fuel clusion is, however, not confirmed by observation. network that is in an early stage of decomposition. (The lack of sensitivity of the rate of burning to The gas-filled space above the fuel network is the compartment temperature can be observed, divided into two zones. Zone l(b) is occupied for example, by comparing in the report of the mainly by air; zone 2(b) consists of a mixture of CIB cooperative program [lo] corresponding volatile decomposition products and combustion values of rate of burning (p. 37-38) and tempera- gases. These two zones are separated by the flame ture (p. 4748.) This fact was brought up earlier envelope (shown by dashed and dotted lines). by the author [ll] as one of the reasons for Air moves rather freely over the collapsed fuel questioning the assumed role of thermal feedback network in zone l(a). As the rate of oxidation of in compartment fires [12].) The implicit recogni- the ember is relatively low, the bulk of the air tion that the rate of pyrolysis of cellulosic fuel passes by this zone without significant depletion shows very little dependence on temperature of its oxygen content. Then, as the air flow conditions made it possible for many laboratories reaches zone 2(a), it separates into two streams. to conduct experimental studies of the rate of One penetrates the fuel network to feed oxidation compartment burning without extensive instru- of the surface char and limited combustion of the mentation. volatile decomposition products within the fuel It is quite obvious by now that the gas-phase network. The other air stream rises with the bulk oxidation reactions cannot play a decisive role in of the volatiles released by the fuel and part of it the mechanism by which compartment ventila- becomes entrained in the flame envelope3. Zone tion controls the rate of pyrolysis of the fuel. 3(a) receives scarcely any oxygen and, therefore, According to the model suggested by the author lacks combustion-induced heat sources. A slow [7] a few years ago, it is the oxidation of the sur- decomposition of fuel takes place under the face char over the decomposing fuel that is instru- effect of thermal feedback (mainly by radiation) mental in controlling pyrolysis by ventilation. from the flame envelope and from the hot gases Unfortunately, this model was not described in and compartment boundaries. sufficient detail and apparently escaped the atten- This model claims that the pyrolysis of fuel tion of subsequent investigators of the subject of is driven, to a great extent, by the heat generated fully developed compartment fires. It seems by oxidation of the char covered surfaces of the desirable, therefore, to present the model with network and that therefore zone 2(a) is by far the more detailed support. largest source of volatile production. (The heat of combustion of the char is about twice that of the SUGGESTED MECHANISM volatile decomposition products.) The important part that the oxidation of char plays in the process Figure 1 is a schematic illustration of a burning of pyrolysis can be established by both observa- compartment at an advanced stage of fire (assumed to be ventilation-controlled). The furniture is modelled as a loosely-packed wood pile uniformly It has been observed [5, 131 that air entrainment is very poor along the horizontal frame envelope (shown distributed on the floor. Part of the pile, that near by dotted line in Fig. I), which represents the lower the window, has already been reduced to ember. boundary of the mixture of volatiles and combustion The lower regions of the compartment are divided products as it moves under the ceiling towards the into three zones. Zone l(a) contains the glowing window. On the other hand, entrainment is quite remains of the collapsed fuel network. Zone 2(a), vigorous along the boundaries of the vertical column of which will be referred to as the zone of intense volatiles (shown by dashed line). Consequently, the height of the compartment has a much more pro- pyrolysis, contains a still erect segment of the fuel nounced effect on the rate of gakphase reactions than network and is characterized by the fact that part have its other dimensions. T. Z. HARMATHY tions and special experimental techniques. In et al., experimenting with different crib arrange- crib burning experiments, spots of glowing char ments [18] and with actual furniture [19] . Some appear soon after ignition, starting at the lower degree of dependence of the rate of burning (py- edges of the outer sticks, and gradually extend rolysis) on the porosity of the fuel bed is, how- over the entire crib surface. That oxidation does ever, suggested by the work of Nilsson [20] and indeed contribute to the glow of char, was con- Thomas and Nilsson [21 ] ). firmed recently by the author [14] by studying As the volatiles are gradually depleted in the the effect of forced ventilation on the rate of zone of intense pyrolysis, the fuel network (furni- burning (pyrolysis) of cribs of wood and various ture in general) collapses and an adjacent section plastics. While the rate of burning of non-charring of the network becomes exposed to the flush of platics was found to be virtually unaffected by the inflowing air. In this way the zone of intense rate of air flow across the crib, that of wood and pyrolysis slowly moves from the window to the charring plastics increased markedly (up to a maxi- back of the compartment. The existence of such mum) with ventilation. This finding reflects the zonal decomposition of fuel has been confirmed known response of the oxidation of carbon to the by observations [lo, 221. flow rate of oxidizer [15]. The important role of As discussed in the introduction, the rate of the across-the-crib ventilation in the burning of production of volatile decomposition pr~duct.~ wood cribs was noted earlier by McCarter and U,, is somewhat different from what is commonly Broido [16] who also observed the lack of response referred to as rate of burning, namely the rate of the burning process to thermal feedback from of loss of mass of the fuel, R(=dC/dt). It has been the flames. shown [7] that the relation between the two is The low thermal conductivity of the surface approximately as follows: char offers partial explanation to the relative insensitivity of the rate of pyrolysis to the temperature prevailing in the compartment (and, in turn, Because, according to the model, the oxidation to the rate of gas-phase reactions). Along those of char is the chief source of the heat that drives surfaces where the char does not take part in the decomposition reactions, it seems reasonable oxidation, it insulates the undecomposed core of to assume that the rate of volatile production is the fuel from heat transfer by radiation. Where proportional to the part of the'surface area of the oxidation takes place, the temperature of the decomposing fuel that is at any particular time char surface usually exceeds that of the environ- covered by oxidizing char5, A, ;i.e ., that ment, so that the role of the hot environment is only a passive one; it "blankets" the glowing surface and thus allows a large portion of the heat of oxidation to be transferred to the fuel core. See Nomenclature. Experiments performed by burning cross-cribs of wood Friedman noted [17] that the fuel configura- (for example, Refs. [23] and [24] and some synthetic tions benefiting most from this blanketing effect polymers [14] revealed that the rate of decomposition are usually those that lose most heat when it is of a batch of fuel is constant for a substantial part of absent. On account of this, and because of the the process, in spite of the fact that the total surface area of the cribs gradually decreases. Consequently, in observed insensitivity of crib fire pyrolysis rates any equation expressing proportionality between the to prevailing temperature conditions, one is led rate of pyrolysis and the surface area of the fuel or any to conclude that the rate of pyrolysis of fuel does portion of it, that area must be regarded as constant. It not depend substantially on either the geometry is convenient to relate such areas to the prefire condi- of the fuel network or the rate of combustion tions. It is incorrect to assume, as some authors do, that the rate of decompostion decreases in proportion to the of the volatile decomposition products in the fuel surface and, therefore, that ventilation-controlled compartment. (Further confirmation of the fires may change at some stage of the process into fuel- former claim comes from the work of Butcher surface-controlled fires. BURNING OF COMPARTMENT FIRES

By combining Eqs. (1) to (5) one obtains the following equation for the rate of loss of the mass of fuel (which, unlike Uv, is a conveniently meas- urable quantity):

Thus the described model of compartment burning and subsequent reasoning confirm that the so- called rate of burning is proportional to the ventilation of the compartment. What makes the Fig. 1. Illustration of zonal burning of cellulosic fuel in model and this line of reasoning even more con- fully developed, ventilation-controlled compartment vincing is the fact that (although applied to fires. ventilation-controlled fires only) they point to the existence of fuel-surface-controlled fires as The experimental finding that R (and with it well. U,,) is approximately constant for a substantial It is obvious from Eq. (4) that a cannot be part of the period of the fully developed fire larger than 1; in other words, that the zone of implies, by virtue of Eq. (2), the approximate intense pyrolysis cannot be larger than the entire constancy of A,. As all oxidizing char surfaces compartment. From this it follows that Eq. (5) is that are effective in volatile production are located applicable only as long as &/Af < 1/C3 and that a within the zone of intense pyrolysis (Fig. I), should be defined as 1 whenever Ua/Af > l/C3. A, can be regarded as proportional to the surface On the other hand, the meaning of the condition area of the fuel in this zone, a = 1 is that the entire fuel network in the compartment is simultaneously involved in the decomposition process. For this situation, after combining Eqs. (1) to (4) (with a = I), one obtains In turn, Afz can be expressed in a formal way as some portion of the total free surface area of the fuel network prior to fire, Af, as

This equation states that if the rate of entry of where a is a variable factor suspected to be a func- air into the compartment is large in relation to the tion of the flow rate of air, U,, and the complexity total free surface area of the fuel (i.e., U,/Af > of the fuel network. 1/C3), the so-called rate of burning will be propor- The task is now to find a logical expression for tional to the free surface area of the fuel. This is a. It seems reasonable to assume that the penetra- the kind of fire that is referred to as fuel-surface- tion of air into the fuel network (and with it a) controlled. is directly proportional to the flow rate of air The results of over 250 full-scale and reduced- and inversely proportional to some variable scale compartment burn tests, reported in ten characteristic of the complexity of the network. publications (cited in Ref. [7]), are plotted in By selecting the free surface area of the fuel Fig. 2 in a normalized form suggested by the network for the latter variable, a can be formu- described model of fully-developed compartment lated as fires. In spite of the significant spread of the experimental points, the existence of two distinct regimes, one ventilation-controlled and the other fuel-surface-controlled, is clearly recognizable. T. Z. HARMATHY

to ventilation-controlled, the second to fuel- surface-controlled fires. Some authors prefer to express rate of burning in terms of the so-called ventilation parameter, A,h1I2 (where A, is the window area and h the window height) instead of the rate of entry of air. If it is assumed that (i) at the onset of fully developed fire the window panes are broken and the full window area is available for ventilation, (ii) the fire compartment is perfectly sealed from other parts of the building, and (iii) no forced ventilation is used, the following equation describes the relation between the rate of entry of air and the ventilation parameter [7] :

Fig. 2. Burning behavior of cellulosic fuel in compartment. where pa is the density of air entering the compartment, and g is acceleration due to gravity. (The spread of the points is imputable to three As the duration of the period of fully devel- factors: (i) the poor reproducibility of the char- oped fire, T, roughly coincides with the period of acteristics of fully-developed compartment fires production of volatile decomposition products, even under controlled laboratory conditions it depends on the portion of the total amount of [25, 261, (ii) the deliberate neglect in the de- fuel in the compartment that becomes volatilized scribed model of a host of variables which de- in the decomposition process and on the rate of monstrably have effect on the rate of burning volatile production. It has been estimated [7] (pyrolysis) at a lower significance level, and (iii) that, on the average, 87.2 per cent of the mass of the lack in many reports of sufficient dimensional fuel prior to fire, Go, will turn into volatile decom- details concerning the fuel, which compelled position products. Thus, utilizing Eq. (I), the the author to estimate the free surface area either duration of the fully developed period of fire can from a general description of the fire load or-in be expressed as the case of fuel-surface-controlled fires-from the duration of fully-developed fire with the aid of Eq. (1 11.1 The "best" values of the constants C1C2 and C3 have been assessed from Fig. 2. They are: After substituting the appropriate expressions C1C2 = 0.00578 and C3 = 26.22. With these for R from Eqs. (6a) and (7a), one obtains, for values Eqs. (6) and (7) can be re-written in the ventilation-controlled fires, following explicit forms:

for fuel-surface-controlled fires,

where the dimension of the constant 0.0062 is kg/m2s, and the first equation obviously relates where the dimension of the constant 151 is m2s/kg. BURNING OF COMPARTMENT FIRES

In Eq. (1 1) the definition of the specific surface Unfortunately, most of the important syn- of fuel has been utilized: thetic polymers pyrolyze without leaving behind carbon-rich solid decomposition products. Obviously, the mechanism of burning described earlier is not applicable to such materials. As they are not covered with a heat-producing oxidizing char layer, the pyrolysis of these materials It must be emphasized that derivation of Eqs. must rely entirely on the heat produced by com- [6a], [7a], [lo], and [ll] without the need for bustion of the gaseous decomposition products heat balance considerations was made possible by within the fuel network and on thermal feedback a peculiarity of cellulosic fuels-the observation from the flames, hot gases, and compartment that their pyrolysis is driven to a great extent by boundaries. Consequently, it is unlikely that their the heat produced by oxidation of their surface rate of pyrolysis can be predicted with sufficient char layer and consequently does not depend on accuracy without an analysis of the heat balance the rate of combustion of the volatile decomposi- over the compartment. tion products in the compartment, at least not to There are other significant differences between the same extent as does pyrolysis of most other cellulosics and non-charring plastics. The stoichio- solid fuels. One should remember that what has metric air requirement is almost three times as been described in this section is only a model. In high for many of the latter materials as for the view of the poor reproducibility of fully-devel- volatiles of cellulosic materials. In addition, many oped compartment fires,'this model describes the (those of high B-numbers [IS]) pyrolyze much rate of burning (pyrolysis) and the duration of faster than wood or paper under similar conditions. fire in terms of three variables, U,, Af, and Go, Consequently, the rate of entry of air may be and deliberately neglects those variables which much less than is needed for the complete com- can influence the process of burning at a lower bustion of the pyrolysis products and a substan- significance level only. That the conclusions are tial part of the gasified fuel must, perforce, burn reasonably accurate has been proved by compari- outside the compartment. This fact is not, how- son with a multitude of experimental findings ever, expected to influence significantly the rate derived from full-scale and reduced-scale burn-out of heat evolution inside the compartment where tests (see Fig. 2) performed on rooms of more or rate of combustion of volatiles depends primarily less conventional dimensions. It remains to be seen on rate of entrainment of air into the flame whether they are also applicable to compartments envelope. of markedly different shape, dimension, and pos- Lacking sufficient experimental information sibly other fuel arrays with much greater or lower one can only speculate on the probable course fraction of fuel surface area exposed to room of fires involving non-charring plastics. As heat radiation. production by combustion of the volatile decomposition products within the fuel network is COMPARTMENT FIRES INVOLVING PLASTICS confined to zone 2(a), which, in addition, is the recipient of strong thermal feedback from the Recent experiments performed by the author vertical section of the flame envelope, one can [14] have indicated that the mechanism of expect that the general pattern of burning will burning of piles of char-forming plastics is analo- not be greatly different from that shown in Fig. 1. gous to that of cellulosic materials. It seems pos- Nevertheless for non-charring plastics (unlike sible, therefore, that Eqs. [6a], [7a], [lo] and cellulosic fuel), under the effect of high radiant [I I], perhaps with different constants, can satis- heat fluxes originating from the hot gases above factorily describe the rate of burning (pyrolysis) and from the compartment boundaries, partici- and duration of compartment fires involving char- pation of zone 3(a) in volatile production is also forming plastics also. expected to be substantial. T. Z. HARMATHY

With regard to rate of pyrolysis, R, the fol- CONCLUSION lowing two a priori arguments can be advanced: (I) Because the intensity of heat supply to the The conventional concept of ventilation-controlled surface of the decomposing fuel strongly depends fires, according to which shortage of air in a com- on temperature conditions in the compartment, partment limits the rate of combustion of fuel, the thermal properties and total area of the com- has been shown to be untenable. Alternative partment lining, which have a bearing on temper- mechanisms by which the rate of entry of air ature, must also have a strong effect on R. might control the rate of pyrolysis of cellulosic (2) Because the rate of heat evolution in the materials have been examined and a plausible compartment relies on the rate of air entrainment model suggested. This model has been proved to into the flame envelope and the efficiency of result in conclusions compatible with current entrainment improves as the ceiling height in- knowledge concerning rate of pyrolysis in both creases, R is expected to depend more on height ventilation-controlled and fuel-surface-controlled than on other dimensions of the compartment. fires. It is pointed out in conclusion that the Observations of films of compartment fires suggested model will probably not be applicable involving plastics and recent studies of liquid to non-charring plastics whose pyrolysis relies on fuel fires by Bullen [27] seem to indicate that for the heat produced by combustion of the volatile fuels susceptible to fast pyrolysis a new type of decomposition products. burning mechanism may become effective. Fires involving such materials may become "pyrolysis- This paper is a contribution from the Division controlled." To understand this kind of mech- of Building Research, National Research Council anism, assume that after a period of quasi-steady- of Canada, and is published with the approval of state burning the rate of pyrolysis in the com- the Director of the Division. partment increases. The increased volatile production will gradually build up the layer of com- NOMENCLATURE bustible gases that floats under the ceiling in the direction of the window. Unable to come in con- A area, m2 tact with air in sufficient quantities, they leave the C constant, various dimensions compartment unburned. As they leave, these g acceleration due to gravity, 9.8 m/s2 gases narrow down that portion of the window G total mass of fuel, kg opening available for entry of air. Since both the h height of window, m amount of air and the space available for air R rate of loss of the mass of fuel (so-called rate entrainment are decreased, the rate of heat evolu- of burning), kg/s tion in the compartment drops off and with it, t time, s temperature. Lower temperatures will naturally U mass flow rate; mass rate of production, kg/s result in reduction of the rate of heat supply to a factor, defined by Eq. (9,dimensionless the fuel surface and, in turn, in a trend towards p density, kg/m3 restoration of the earlier level of rate of pyrolysis. 7 duration of fully developed fire, s There is a superficial similarity between pyroly- cp specific surface of fuel, m2/kg sis-controlled fires of plastics and ventilation- controlled fires of cellulosic materials. In both the rate of pyrolysis seems to be independent of, or Subscripts only weakly dependent on, the total fire load and a of air specific surface of the fuel. Yet there are basic c of surface covered with char differences. The most important is that in pyroly- f of fuel sis-controlled fires temperature is an essential o prior to fire link between pyrolysis and ventilation, whereas v of volatile decomposition products in ventilation-controlled fires the relation between w of the window pyrolysis and ventilation is direct. z in the zone of intense pyrolysis BURNING OF COMPARTMENT FIRES

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